A dragline is a type of excavation equipment commonly used in the mining industry. Dragline equipment conventionally includes a bucket for entering the surface of the earth, and for carrying the removed material to the desired location. In a conventional dragline, a hoist rope suspends the bucket from the tip of a boom, and a drag rope, connected near the foot of the boom, operates to drag the bucket horizontally along the ground and, via dump linkage including a pulley, controls the angle of the bucket to control the digging, carrying and dumping operations. In modern dragline equipment, the capacity of the bucket can range from on the order of 10 cubic yards to on the order of 150 cubic yards, with the total weight of a large, fully loaded, bucket capable of exceeding 100 tons. The dragline itself is generally rated for a specific allowable load that it can lift without exceeding its design limit. This rated weight includes the total weight of all components under the boom, including wire rope, chains, and the empty bucket. The actual weight of the material being removed depends, of course, on the density of the removed material, and also the volume removed (which, in turn, depends upon the digging technique, and the angle between the surface of the material and that of the bucket during digging).
In a mining operation using such a dragline, highest efficiency is of course achieved by moving the most earth material in the shortest period of time. Particularly for jobs at which the dragline is operating twenty-four hours a day, dragline downtime is particularly costly. It has been observed that an important factor in the efficiency of a dragline operation is the matching of the bucket design capacity to the job being done. If the bucket is too large for the density of the earth being excavated (or, in other words, has too large a volume capacity relative to the dragline allowable load), failure of the dragline can occur, causing downtime and increasing the cost of the job. Conversely, if the bucket is undersized, in volume capacity, for the density of the removed material, productivity is lost since the full capability of the dragline is not being utilized.
In recent years, dragline buckets of light-weight design have been used in order to increase the amount of material excavated for a dragline of a given allowable load. This incremental decrease in strength provided by such light-weight bucket designs reduces the margin for error in loading the bucket relative to its design limit.
There are many other factors which determine the capacity of a dragline bucket in removing portions of the earth. For example, different bucket manufacturers may use different construction techniques in designing and manufacturing dragline buckets, resulting in different empty weights for the buckets. Furthermore, field modifications to buckets are often made to change the stiffness or durability of the dragline bucket, or to make necessary repairs. Furthermore, different draglines may use different weight chains and lines, which affect the total suspended weight of the dragline rigging and, in turn, the amount of material which may be removed in a single bucketload. In addition, different portions of the surface under excavation may consist of multiple formations of differing densities, such that the bucket weight varies from one dig operation to the next. The angle of repose of the material being dug, and the operator's technique for filling the bucket, also causes the bucket weight to vary from dig to dig, as the material being removed can easily heap in the bucket, and thus exceed the rated capacity of the bucket.
As a result, it is difficult to model or otherwise estimate the full weight of a dragline bucket prior to an excavation operation in such a manner to determine that the bucket is properly sized for the allowable load capacity of the dragline, much less with confidence that it will remain properly sized during the excavation process. Accordingly, it is useful to monitor the actual bucket and rigging weight during the excavation process so that the operator can take real-time corrective action such as modifying the digging parameters, or changing buckets, so as to improve the excavation efficiency. For example, the operator may use different sized buckets for different areas of the mining pit being excavated.
Prior techniques have been used to monitor dragline bucket weights. For example, mechanical and electronic scales have been used to directly measure the weight of the full bucket load. However, cessation of the digging operation to weigh bucket loads adds overhead time to the overall excavation to such a degree that such weighing of the bucket would be done, at best, for sample loads, rather than for all loads. In addition, dragline buckets are quite large, with dimensions that vary widely from bucket to bucket, making it quite impractical to move a fully loaded bucket to a conventional scale. Attempts to use portable scales to weigh loaded dragline buckets have met with limited success.
Another prior technique involves the recordation of operating parameters, by way of a strip chart recorder or the like, coded into machine-readable form for analysis by an off-site computer, for example using a running average technique. Such a technique, however, fails to provide real-time on-site process control information, as the analysis is performed after the excavation. In addition, this method discounts abnormal conditions encountered by the dragline, as would occur during normal operation of a dragline.
Furthermore, it should be noted that post-excavation analysis of the data is done without the benefit of on-site visual information. It should be noted that such events as loss of material from the bucket during the raising of the bucket, bouncing of the bucket after being raised, and the like, each of which cause the calculated bucket weight data to be in error, will not necessarily be apparent from the recorded data. As such, invalid data which ought to be discarded or discounted is used in the calculation of the bucket weight in such post-excavation analysis.
Furthermore, it should be noted that this prior method has utilized hoist armature current from one motor, even for draglines where more than one hoist motor is included. Even with optimized balancing of multiple hoist motors in a dragline, it has been observed that the actual pulling force of the hoist motors will not be equally distributed among the multiple hoist motors. Accordingly, calculation of the bucket weight using the armature current from only one of the motors will introduce error into the calculation.
Other prior techniques have been used to monitor and report various excavation parameters. A first of these prior techniques is the DIGMATE.RTM. Plus On-Board Excavator Monitor and Production System (including the BOOMSENTRY.RTM. Plus Anti-Tightline Control), manufactured and sold by General Electric. In this permanently installed system, the dragline power, hoist rope force, drag rope force, hoist motor field strength, hoist rope position, drag rope position, and swing angle parameters are continuously measured and monitored, with thirty-two dragline parameters calculated therefrom, including bucket fill out to dump and bucket fill return to dig, both measured in tons.
As in the off-line analysis case, data collected continuously will necessarily provide a substantial quantity of data which ought to be discarded from a determination of bucket weight, such as resulting from cycles with loss of material, bouncing, or other bucket transient conditions. To avoid this problem, it is believed that permanently installed monitoring systems generally include an algorithm based on generalized assumptions about the dig cycle operation, which determines the time and duration of data collection during each the dig cycle. However, generalization of the dig cycles will necessarily be invalid for certain operations (for example, where the duration of data collection is longer than the time at which the load is suspended), and also limits the system's ability to take advantage of those operations for which larger amounts of data may be taken. Accordingly, it is believed that the accuracy of the bucket weight measurements from permanently installed systems is limited.
Another prior system is described in U.S. Pat. No. 4,677,579, issued to D. Radomilovich on Jun. 20, 1987. The disclosed system is particularly adapted to a shovel loader (although, as noted in column 1, lines 6 through 10, and in column 2, lines 9 through 14, this system is disclosed as applicable to a dragline). Parameters relative to the operation of the hoist motor, such as armature voltage, field current, and RPM are measured and, together with parameters concerning the geometry of the system, are used to analytically compute the total weight of the bucket using the dynamic calculation of dividing a force by an acceleration; the reference also indicates that calculation of the bucket weight using the total vertical force as an apparent weight is known (see column 7, lines 15 through 24). However, this method is of course quite complicated by the inclusion of the necessary data and calculations to analytically determine the load weight, taking into account acceleration, deceleration, and other transient effects. Furthermore, it is believed that the bucket weight of a dragline can be more accurately calculated in substantially a steady-state condition, relative to the more dynamic condition disclosed in this reference.
It is therefore an object of this invention to provide a system and method of providing real-time measurement of a suspended weight, such as a dragline bucket weight.
It is a further object of this invention to provide such a system and method which can be performed without cessation of the digging operation.
It is a further object of this invention to provide such a system and method which utilizes visual information during the operation so that transients and dynamic effects are minimized.
It is a further object of this invention to provide such a system and method which utilizes visual information so that improper data is not taken or utilized in the calculation of the suspended weight.
It is a further object of this invention to provide such a method and system which provides correction for parasitic effects, such as friction, of the hoist motor, gear train, and rope sheaves, based on actual field data rather than on modeled or theoretical estimates.
It is a further object of this invention to provide such a method and system which uses both hoist field and hoist armature currents, from each of several hoist motors, to determine the suspended weight.
It is a further object of this invention to provide such a method and system which provides summary output listing the bucket weights of a number of dig cycles measured during the excavation.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to this specification, together with the drawings.